2. Chapter Overview
• Stack Operations
• Defining and Using Procedures
• Program Design Using Procedures
3. Link Library Overview
• A file containing procedures that have
been compiled into machine code
– constructed from one or more OBJ files
• To build a library, . . .
– start with one or more ASM source files
– assemble each into an OBJ file
– create an empty library file (extension .LIB)
– add the OBJ file(s) to the library file, using
the Microsoft LIB utility
4. Calling a Library Procedure
• Call a library procedure using the CALL instruction.
Some procedures require input arguments. The
INCLUDE directive copies in the procedure
prototypes (declarations).
• The following example displays "1234" on the
console: Irvine32.inc
INCLUDE
.code
mov eax,1234h ; input argument
call WriteHex ; show hex number
call Crlf ; end of line
5. Linking to a Library
• Your programs link to Irvine32.lib using the linker
command inside a batch file named make32.bat.
• Notice the two LIB files: Irvine32.lib, and
kernel32.lib
– the latter is part of the Microsoft Win32 Software
Development Kit (SDK)
6. Library Procedures - Overview
CloseFile – Closes an open disk file
Clrscr - Clears console, locates cursor at upper left corner
CreateOutputFile - Creates new disk file for writing in output mode
Crlf - Writes end of line sequence to standard output
Delay - Pauses program execution for n millisecond interval
DumpMem - Writes block of memory to standard output in hex
DumpRegs – Displays general-purpose registers and flags (hex)
GetCommandtail - Copies command-line args into array of bytes
GetMaxXY - Gets number of cols, rows in console window buffer
GetMseconds - Returns milliseconds elapsed since midnight
7. Library Procedures - Overview
(cont.)
GetTextColor - Returns active foreground and background text colors in
the console window
Gotoxy - Locates cursor at row and column on the console
IsDigit - Sets Zero flag if AL contains ASCII code for decimal digit (0–9)
MsgBox, MsgBoxAsk – Display popup message boxes
OpenInputFile – Opens existing file for input
ParseDecimal32 – Converts unsigned integer string to binary
ParseInteger32 - Converts signed integer string to binary
Random32 - Generates 32-bit pseudorandom integer in the range 0 to
FFFFFFFFh
Randomize - Seeds the random number generator
RandomRange - Generates a pseudorandom integer within a specified
range
ReadChar - Reads a single character from standard input
8. Library Procedures - Overview
(cont.)
ReadFromFile – Reads input disk file into buffer
ReadDec - Reads 32-bit unsigned decimal integer from keyboard
ReadHex - Reads 32-bit hexadecimal integer from keyboard
ReadInt - Reads 32-bit signed decimal integer from keyboard
ReadKey – Reads character from keyboard input buffer
ReadString - Reads string from standard input, terminated by [Enter]
SetTextColor - Sets foreground and background colors of all
subsequent console text output
StrLength – Returns length of a string
WaitMsg - Displays message, waits for Enter key to be pressed
WriteBin - Writes unsigned 32-bit integer in ASCII binary format.
WriteBinB – Writes binary integer in byte, word, or doubleword format
WriteChar - Writes a single character to standard output
9. Library Procedures - Overview
(cont.)
WriteDec - Writes unsigned 32-bit integer in decimal format
WriteHex - Writes an unsigned 32-bit integer in hexadecimal
format
WriteHexB – Writes byte, word, or doubleword in hexadecimal
format
WriteInt - Writes signed 32-bit integer in decimal format
WriteString - Writes null-terminated string to console window
WriteToFile - Writes buffer to output file
WriteWindowsMsg - Displays most recent error message
generated by MS-Windows
10. Example 1
Clear the screen, delay the program for 500 milliseconds,
and dump the registers and flags.
.code
call Clrscr
mov eax,500
call Delay
call DumpRegs
Sample output:
EAX=00000613 EBX=00000000 ECX=000000FF EDX=00000000
ESI=00000000 EDI=00000100 EBP=0000091E ESP=000000F6
EIP=00401026 EFL=00000286 CF=0 SF=1 ZF=0 OF=0
11. Example 2
Display a null-terminated string and move the cursor
to the beginning of the next screen line.
.data
str1 BYTE "Assembly language is easy!",0
.code
mov edx,OFFSET str1
call WriteString
call Crlf
12. Example 3
Display an unsigned integer in binary, decimal, and
hexadecimal, each on a separate line.
IntVal = 35
.code
mov eax,IntVal
call WriteBin ; display binary
call Crlf
call WriteDec ; display decimal
call Crlf
call WriteHex ; display hexadecimal
call Crlf
Sample output:
0000 0000 0000 0000 0000 0000 0010 0011
35
23
13. Example 4
Input a string from the user. EDX points to the string
and ECX specifies the maximum number of characters
the user is permitted to enter.
.data
fileName BYTE 80 DUP(0)
.code
mov edx,OFFSET fileName
mov ecx,SIZEOF fileName – 1
call ReadString
A null byte is automatically appended to the string.
14. Example 5
Generate and display ten pseudorandom signed integers
in the range 0 – 99. Pass each integer to WriteInt in EAX
and display it on a separate line.
.code
mov ecx,10 ; loop counter
L1: mov eax,100 ; ceiling value
call RandomRange ; generate random int
call WriteInt ; display signed int
call Crlf ; goto next display line
loop L1 ; repeat loop
15. Example 6
Display a null-terminated string with yellow characters
on a blue background.
.data
str1 BYTE "Color output is easy!",0
.code
mov eax,yellow + (blue * 16)
call SetTextColor
mov edx,OFFSET str1
call WriteString
call Crlf
The background color is multiplied by 16 before being added
to the foreground color.
16. Stack Operations
• Runtime Stack
• PUSH Operation
• POP Operation
• PUSH and POP Instructions
• Using PUSH and POP
• Example: Reversing a String
• Related Instructions
17. Runtime Stack
• Imagine a stack of plates . . .
– plates are only added to the top
– plates are only removed from the top
– LIFO (Last-In, First-Out) structure
– Push & pop operations
18. Runtime Stack
• Managed by the CPU, using two registers
– SS (stack segment)
– ESP (stack pointer) * SS
* SP in Real-address mode
19. PUSH Operation
• A 32-bit push operation decrements the
stack pointer by 4 and copies a value into
the location pointed to by the stack pointer.
PUSH 0A5h
20. PUSH Operation (cont.)
• Same stack after pushing two more
integers:
The stack grows downward. The area below ESP is
always available (unless the stack has overflowed).
21. POP Operation
• Copies value at stack[ESP] into a register or variable.
• Adds n to ESP, where n is either 2 or 4.
– value of n depends on the attribute of the operand
receiving the data
Pop EAX
EAX = 00000002
22. PUSH and POP Instructions
• PUSH syntax:
– PUSH r/m16
– PUSH r/m32
– PUSH imm32
• POP syntax:
– POP r/m16
– POP r/m32
23. When to Use Sacks
• To save and restore registers
• To save return address of a procedure
• To pass arguments
• To support local variables
24. Example: Using PUSH and
POP
Save and restore registers when they contain important
values. PUSH and POP instructions occur in the opposite
order.
push esi ; push registers
push ecx
push ebx
mov esi,OFFSET dwordVal ; display some memory
mov ecx,LENGTHOF dwordVal
mov ebx,TYPE dwordVal
call DumpMem
pop ebx ; restore registers
pop ecx ; opposite order
pop esi
25. Example: Nested Loop
When creating a nested loop, push the outer loop
counter before entering the inner loop:
mov ecx,100 ; set outer loop count
L1: ; begin the outer loop
push ecx ; save outer loop count
mov ecx,20 ; set inner loop count
L2: ; begin the inner loop
;
;
loop L2 ; repeat the inner loop
pop ecx ; restore outer loop count
loop L1 ; repeat the outer loop
26. Related Instructions
• PUSHFD and POPFD
– push and pop the EFLAGS register
• PUSHAD pushes the 32-bit general-purpose
registers on the stack
– order: EAX, ECX, EDX, EBX, ESP, EBP, ESI,
EDI
• POPAD pops the same registers off the
stack in reverse order
– PUSHA and POPA do the same for 16-bit
registers
27. Example: Reversing String
.data
aName BYTE "Abraham Lincoln",0
nameSize = ($ - aName) – 1
.code
main PROC
; Push the name on the stack.
mov ecx,nameSize
mov esi,0
L1:
movzx eax,aName[esi] ; get character
push eax ; push on stack
inc esi
Loop L1
28. Example: Reversing String
(cont.)
; Pop the name from the stack, in reverse,
; and store in the aName array.
mov ecx,nameSize
mov esi,0
L2:
pop eax ; get character
mov aName[esi],al ; store in string
inc esi
Loop L2
exit
main ENDP
END main
29. Defining and Using Procedures
• Creating Procedures
• Documenting Procedures
• Example: SumOf Procedure
• CALL and RET Instructions
• Nested Procedure Calls
• Local and Global Labels
• Procedure Parameters
• Flowchart Symbols
• USES Operator
30. Creating Procedures
• Procedure
– A named block of statements that ends in a return statement
• Large problems can be divided into smaller tasks to make them
more manageable
• A procedure is the ASM equivalent of a Java or C++ function
• Following is an assembly language procedure named sample:
sample PROC
.
.
ret
sample ENDP
31. Documenting Procedures
Suggested documentation for each procedure:
• A description of all tasks accomplished by the
procedure.
• Receives: A list of input parameters; state their usage
and requirements.
• Returns: A description of values returned by the
procedure.
• Requires: Optional list of requirements called
preconditions that must be satisfied before the procedure
is called.
If a procedure is called without its preconditions satisfied, it will
probably not produce the expected output.
32. Example: SumOf Procedure
;---------------------------------------------------------
SumOf PROC
;
; Calculates and returns the sum of three 32-bit integers.
; Receives: EAX, EBX, ECX, the three integers. May be
; signed or unsigned.
; Returns: EAX = sum, and the status flags (Carry,
; Overflow, etc.) are changed.
; Requires: nothing
;---------------------------------------------------------
add eax,ebx
add eax,ecx
ret
SumOf ENDP
33. CALL and RET Instructions
• The CALL instruction calls a procedure
– pushes offset of next instruction on the stack
– copies the address of the called procedure into EIP
ESP = ESP - 4 ; push return address
SS:ESP = EIP ; onto the stack
EIP = EIP + relative offset (or displacement)
; update EIP to point to procedure
• The RET instruction returns from a procedure
– pops top of stack into EIP
EIP = SS:ESP ; pop return address
ESP = ESP + 4 ; from the stack
34. CALL-RET Example
main PROC
00000020 call MySub
0000025 is the offset of 00000025 mov eax,ebx
the instruction .
immediately following .
the CALL instruction main ENDP
MySub PROC
00000040 is the offset 00000040 mov eax,edx
of the first instruction .
inside MySub .
ret
MySub ENDP
35. CALL-RET Example (cont.)
The CALL
instruction pushes
00000025 onto the
stack, and loads
00000040 into EIP
The RET
instruction pops
00000025 from the
stack into EIP
(stack shown before RET executes)
36. Nested Procedure Calls
By the time Sub3 is called, the
stack contains all three return
addresses:
37. How Is Program Control
Transferred?
Offset(hex) machine code(hex)
main:
. . . .
00000002 E816000000 call sum
00000007 89C3 mov EBX,EAX
. . . .
; end of
main procedure
sum:
0000001D 55 push EBP
. . . .
; end of sum
procedure
avg:
. . . .
00000028 E8F0FFFFFF call sum
38. Local and Global Labels
A local label is visible only to statements inside the
same procedure. A global label is visible everywhere.
main PROC
jmp L2 ; error
L1:: ; global label
exit
main ENDP
sub2 PROC
L2: ; local label
jmp L1 ; ok
ret
sub2 ENDP
39. Procedure Parameters
• A good procedure might be usable in many
different programs
– but not if it refers to specific variable names
• Parameters help to make procedures flexible
because parameter values can change at
runtime
40. Parameter Passing
Mechanisms
• Call-by-value
– Receives only values
– Similar to mathematical functions
• Call-by-reference
– Receives pointers
– Directly manipulates parameter storage
41. Parameter Passing
• Parameter passing is different and complicated than in a high-
level language
• In assembly language
– You should first place all required parameters in a mutually
accessible storage area
– Then call the procedure
• Types of storage area used
– Registers (general-purpose registers are used)
– Memory (stack is used)
• Two common methods of parameter passing:
– Register method
– Stack method
42. Parameter Passing: Register
Method
The ArraySum procedure calculates the sum of an array. It
makes two references to specific variable names:
ArraySum PROC Call-by-reference
mov esi,0 ; array index
mov eax,0 ; set the sum to zero
mov ecx,LENGTHOF myarray ; set number of elements
L1: add eax,myArray[esi] ; add each integer to sum
add esi,4 ; point to next integer
loop L1 ; repeat for array size
mov theSum,eax ; store the sum
ret
ArraySum ENDP
What if you wanted to calculate the sum of two or
three arrays within the same program?
43. Procedure Parameters (cont.)
This version of ArraySum returns the sum of any
doubleword array whose address is in ESI. The sum is
returned in EAX:
ArraySum PROC
; Receives: ESI points to an array of doublewords,
; ECX = number of array elements.
; Returns: EAX = sum
;-----------------------------------------------------
mov eax,0 ; set the sum to zero
L1: add eax,[esi] ; add each integer to sum
add esi,4 ; point to next integer
loop L1 ; repeat for array size
ret
ArraySum ENDP
45. USES Operator
• Lists the registers that will be preserved
ArraySum PROC USES esi ecx
mov eax,0 ; set the sum to zero
etc.
MASM generates the code shown in blue:
ArraySum PROC
push esi
push ecx
.
.
pop ecx
pop esi
ret
ArraySum ENDP
46. When Not to Push a Register
The sum of the three registers is stored in EAX on line
(3), but the POP instruction replaces it with the starting
value of EAX on line (4):
SumOf PROC ; sum of three integers
push eax ; 1
add eax,ebx ; 2
add eax,ecx ; 3
pop eax ; 4
ret
SumOf ENDP
Call-by-value
SumOf PROC ; sum of three integers
add eax,ebx ; 2
add eax,ecx ; 3
ret
SumOf ENDP
47. Pros and Cons of the Register
Method
• Advantages
– Convenient and easier
– Faster
• Disadvantages
– Only a few parameters can be passed using
the register method
– Only a small number of registers are available
– Often these registers are not free
– freeing them by pushing their values onto the stack
negates the second advantage
48. Parameter Passing: Stack
Method
• All parameter values are pushed onto the
stack before calling the procedure
• Example:
push number1
push number2
call sum
49. Accessing Parameters on the
Stack
• Parameter values are buried inside the
stack
• We can use the following to read
number2
mov EBX,[ESP+4]
Problem: The ESP value changes with
push and pop operations
• Relative offset depends of the stack
operations performed
• Is there a better alternative?
50. Using BP Register to Access Parameters
• Preferred method of accessing parameters on the stack is
mov EBP,ESP
mov EAX,[EBP+4]
to access number2 in the previous example
• Problem: BP contents are lost!
– We have to preserve the contents of BP
– Use the stack (caution: offset value changes)
push EBP
mov EBP,ESP
51. Clearing the Stack Parameters
Stack state after Stack state after Stack state after
saving EBP pop EBP executing ret
52. Clearing the Stack Parameters (cont.)
• Two ways of clearing the unwanted parameters on the stack:
– Use the optional-integer in the ret instruction
• in the previous example, you can use
ret 4
EIP = SS:ESP
ESP = ESP + 4 + optional-integer
– Add the constant to ESP in calling procedure (C uses this
method)
push number1
push number2
call sum
add ESP,4
53. Housekeeping Issues
• Who should clean up the stack of unwanted
parameters?
– Calling procedure
• Need to update ESP with every procedure call
• Not really needed if procedures use fixed number of
parameters
• C uses this method because C allows variable
number of parameters
– Called procedure
• Code becomes modular (parameter clearing is done
in only one place)
• Cannot be used with variable number of parameters
54. Housekeeping Issues (cont.)
• Need to preserve the state (contents of the registers) of the
calling procedure across a procedure call.
• Stack is used for this purpose
• Which registers should be saved?
– Save those registers that are used by the calling procedure
but are modified by the called procedure
• Might cause problems as the set of registers used by the
calling and called procedures changes over time
– Save all registers (brute force method) by using pusha
• Increased overhead (pusha takes 5 clocks as opposed 1 to
save a register)
55. Housekeeping Issues (cont.)
• Who should preserve the state of the calling
procedure?
– Calling procedure
• Need to know the registers used by the called
procedure
• Need to include instructions to save and restore
registers with every procedure call
• Causes program maintenance problems
– Called procedure
• Preferred method as the code becomes modular (state
preservation is done only once and in one place)
• Avoids the program maintenance problems mentioned
59. Program Design Using Procedures
• Top-Down Design (functional decomposition)
involves the following:
– design your program before starting to code
– break large tasks into smaller ones
– use a hierarchical structure based on procedure
calls
– test individual procedures separately
60. Integer Summation Program
Description: Write a program that prompts the
user for multiple 32-bit integers, stores them in
an array, calculates the sum of the array, and
displays the sum on the screen.
Main steps:
• Prompt user for multiple integers
• Calculate the sum of the array
• Display the sum
63. PromptForIntegers
PromptForIntegers PROC
;
; Prompts the user for an array of integers, and
; fills the array with the user's input.
; Receives: ESI points to the array,
; ECX = array size
; Returns: nothing
;--------------------------------------------------
pushad ; save all registers
mov edx,OFFSET prompt1 ; address of the prompt
cmp ecx,0 ; array size <= 0?
jle L2 ; yes: quit
64. PromptForIntegers
L1:
call WriteString ; display string
call ReadInt ; read integer into EAX
call Crlf ; go to next output line
mov [esi],eax ; store in array
add esi,4 ; next integer
loop L1
L2:
popad ; restore all registers
ret
PromptForIntegers ENDP
65. DisplaySum
DisplaySum PROC
; Displays the sum on the screen
; Receives: EAX = the sum
; Returns: nothing
;---------------------------------------------------
push edx
mov edx,OFFSET prompt2 ; display message
call WriteString
call WriteInt ; display EAX
call Crlf
pop edx
ret
DisplaySum ENDP
66. Code Fragment
IntegerCount = 3 ; array size
.data
prompt1 BYTE "Enter a signed integer: ",0
prompt2 BYTE "The sum of the integers is: ",0
array DWORD IntegerCount DUP(?)
.code
main PROC
call Clrscr
mov esi,OFFSET array
mov ecx,IntegerCount
call PromptForIntegers
call ArraySum
call DisplaySum
exit
main ENDP
